This invention relates to an apparatus for scanning a plane with light beams and, more particularly, to a laser printer system for scanning a photosensitive surface with a plurality of laser beams.
The laser printer system can print a pattern or a picture at a high speed and with high precision, and there have been developed various types of systems. The scanning apparatus of the laser printer system has a construction as shown in FIG. 1. A laser unit 11, e.g., a semiconductor laser unit, emits a diverged laser beam, which is collimated by a collimator lens 12 into a parallel laser beam directed to a convergent lens 13. The convergent lens 13 diverges the collimated laser beam, and the diverged laser beam is incident on a rotating polygonal mirror 14 to be reflected by a reflecting surface thereof. The reflected and diverged laser beam is projected onto a surface 15, i.e., a photosensitive surface, as it is deflected by the polygonal mirror 14. The surface 15 is thus scanned by the diverged laser beam as shown in FIG. 2. When the surface 15 is scanned by a light intensity modulated laser beam, a pattern or picture image drawn by the modulated laser beam is formed on the photosensitive surface 15. In the scanning apparatus of this type, the laser unit 11, collimator lens 12, convergent lens 13, polygonal mirror 14 and surface 15 are arranged such that the image of a point light source of the laser unit 11 is formed on the surface 15.
In the scanning apparatus shown in FIG. 1, the rotational speed of the polygonal mirror 14 is substantially proportional to the scanning speed of the laser beam. To increase the scanning speed, therefore, the rotational speed of the polygonal mirror 14 must be increased. However, there is an upper limit of the rotational speed at which the polygonal mirror can be rotated stably and accurately, and increasing it beyond the limit is liable to lower the quality of the printed picture or pattern. Further, in the scanning apparatus of FIG. 1 the polygonal mirror 14 must be fabricated with high precision. Therefore, its costs is high and leads to a high cost of the scanning apparatus. With the scanning apparatus of FIG. 1, a picture actually can be reproduced with a sufficiently high resolution if the polygonal mirror 14 is rotated at a low speed while the surface is scanned at a low speed.
In order to solve the problems in the scanning apparatus as mentioned above, there has been proposed a scanning apparatus, which can scan the surface 15 with a plurality of laser beams as shown in FIG. 3. To scan the surface 15 with a plurality of laser beams at a time, is equivalent to the scanning of the surface with a single laser beam at a high speed. FIG. 4 shows an example of the scanning apparatus which scans the surface 15 with a plurality of laser beams. The apparatus comprises first to third laser beams 11A to 11C for emitting first to third laser beams respectively. The first to third laser beams are collimated by respective first to third collimator lens 12A to 12C. The second collimated laser beam is directed toward a convergent lens so that it is incident thereon directly. The first parallel laser beam, on the other hand, is reflected by mirrors 16A and 16B so that it is placed in an optical path, which extends parallel to and in the close proximity of the optical path of the first parallel laser beam to the convergent lens. Likewise, the third parallel laser beam is reflected by mirrors 18A and 18B. The first to third laser beams are thus incident as parallel light rays on the convergent lens. The three laser beams incident in parallel with one another and in the close proximity of one another on the convergent lens, are converged to be incident on a reflecting surface of a rotating polygonal mirror. The laser beams reflected by the rotating polygonal mirror are incident on the surface. The surface is thus scanned by the three converged laser beams at a time. With this scanning apparatus, a greater area of the surface can be scanned when the polygonal mirror is rotated at the same speed as the mirror shown in FIG. 1. That is, the rotational speed of the polygonal mirror and the scanning speed can be reduced for scanning the surface substantially in the same period of time of high speed scanning by the apparatus of FIG. 1.
In the apparatus of FIG. 4, however, it is required to provide the collimator lenses corresponding in number to the number of the laser units and also the mirrors for re-directing the first and third parallel laser beams to place them in respective predetermined optical paths. That is, the number of optical components of the scanning apparatus is increased. Besides, the laser units, collimator lenses and mirrors must be arranged accurately relative to one another. This means that the adjustment of the optical system is more cumbersome and requires a great deal of extra time.